The present invention relates to a receiver for the reception of scrambled signals, which signals are to be subjected to a descrambling process under the control of control words conveyed from a conditional access sub-system coupled to said receiver, said control words being produced in response to data conveyed from said receiver to said sub-system, a fresh control word being required at intervals for performing the descrambling process during a given period.
In the evolution of the specifications for MAC-packet television transmission systems which are currently represented by the European Broadcasting Union documents "Television Standards for the Broadcasting Satellite Service--Specification of the C-MAC/Packet System"--SPB 284, 3rd revised version published December 1984 and "Methods of Conveying C-MAC/Packet Signals in Small and Large Community Antenna and Cable Network Installations"--SPB 352 published December 1984, which documents are incorporated herein by way of reference, the requirement to scramble vision, sound and data services for conditional access purposes was identified as an important requirement at an early stage. For this reason, all members of the MAC-packet family permit the generation of the control words required for descrambling in response to data received by the receiver. With the MAC-packet family, a new control word is required at intervals of 256 television frames for generating the data needed to provide the descrambling process in the receiver during a given period of 256 television frames. Obviously, the control word required for a given period needs to be received from the sub-system prior to the start of the given period and while this is perfectly satisfactory during the reception of a service, problems may be encountered upon selecting a service (such as at switch on), or if there should be an interruption of the service. This could result in a scrambled signal not being descrambled for an entire descrambling period of approximately 10 seconds (256 frames at 25 frames per second).